Merge #16129: refactor: Remove unused includes

67f4e9c522 Include core_io.h from core_read.cpp (practicalswift)
eca9767673 Make reasoning about dependencies easier by not including unused dependencies (practicalswift)

Pull request description:

  Make reasoning about dependencies easier by not including unused dependencies.

  Please note that the removed headers are _not_ "transitively included" by other still included headers. Thus the removals are real.

  As an added bonus this change means less work for the preprocessor/compiler. At least 51 393 lines of code no longer needs to be processed:

  ```
  $ git diff -u HEAD~1 | grep -E '^\-#include ' | cut -f2 -d"<" | cut -f1 -d">" | \
        sed 's%^%src/%g' | xargs cat | wc -l
  51393
  ```

  Note that 51 393 is the lower bound: the real number is likely much higher when taking into account transitively included headers :-)

ACKs for commit 67f4e9:

Tree-SHA512: 0c8868aac59813f099ce53d5307eed7962dd6f2ff3546768ef9e5c4508b87f8210f1a22c7e826c3c06bebbf28bdbfcf1628ed354c2d0fdb9a31a42cefb8fdf13

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